[FFmpeg-trac] #11056(avformat:closed): Position of key frame incorrect

[FFmpeg]

#11056: Position of key frame incorrect
-------------------------------------+-------------------------------------
             Reporter:  David        |                    Owner:  (none)
  Johansen                           |
                 Type:  defect       |                   Status:  closed
             Priority:  normal       |                Component:  avformat
              Version:  6.1.1        |               Resolution:  invalid
             Keywords:  ffprobe pos  |               Blocked By:
  mp4                                |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by David Johansen):

 Replying to [comment:6 Balling]:
 > Use hex editor and find out what is the offset.
 >
 > Ib sill do not get why does this matter. What is your need to know the
 perfect offset of last keyframe?
 >
 > Does it happen on other keyframes not in the end (on other files)?

 Yes, I will find an example where it happens in the middle of the file

 Replying to [comment:7 MasterQuestionable]:
 > ͏    No wonder then...
 > ͏    The MP4 format may be a bit more (or much more...) complicated than
 that: mere concatenation may not properly present.
 >
 > ͏    In particular, it has the concepts similar to Matroska's Cluster.

 I misrepresented what's being done slightly. These are fragmented mp4
 files that are being made by the HLS muxer that I'm then combining into a
 single file so they're simpler to store/serve, but I need to know the
 location of the keyframes to be able to provide the necessary
 `EXT-X-BYTERANGE` info

 Replying to [comment:9 mkver]:
 > There is 0x00015827 at offset 931036. This corresponds to a NAL unit
 with size 88103 which fits nicely with the packet size 88107 as reported
 by ffprobe. There is 0x0000047C at offset 930976. This is already invalid
 with a NALU length size of two. With a NALU length size of three, this
 would be a NALU with nal_unit_type 28 -- one of the unspecified ones. The
 next NALU would then have a size of 0x340000, which is larger than the
 whole file. With a nal unit length size of four, the next NALU has a
 length of more than 2GiB.
 >
 > To sum it up, your offset 930976 (for which you provided no real
 explanation) is just wrong.

 Using the values that I provided for `EXT-X-BYTERANGE` it will playback in
 Video.js and Safari, but using the values that `ffprobe` provides it
 stops, so that's my current criteria for deciding if it's "correct"
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/11056#comment:10>
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